This invention relates to mechanical springs and more particularly relates to improvements in a particular type of mechanical spring that has been used in the past for resonating the displacer of a free piston Stirling engine or cooler within its casing but its improvement permits the spring to resonate multiple different bodies instead of only one.
The term “flexure bearing spring” is adopted to refer to a type of spring that has been referred to in the prior art by various terms, including planar spring, flexure bearing, and flat spiral spring. Examples of such springs are shown in U.S. Pat. Nos. 5,522,214 and 8,028,409 and U.S. published application US 2008/0191399. Flexure bearing springs are commonly used to support reciprocating bodies such as the piston and displacer of a free piston Stirling engine or cooler. A typical flexure bearing spring has a surrounding peripheral frame with a central axis that is perpendicular to the frame and a set of arms extending inwardly from the peripheral frame. The arms do not extend inwardly along a radial of the axis but instead each arm progresses inwardly along a more or less spiral-looking path. The inner ends of the arms are ordinarily joined together within the peripheral frame where they are attached to a body that reciprocates along the central axis. The flexure bearing spring is a bearing in the sense that it supports the body against radial movement away from its axis of reciprocation. It is a flexure in the sense that its arms are cantilevered springs that flex during reciprocation of the body. It is a spring in the sense that its arms are elastic and resilient and therefore can receive, store and return energy. The arms are equi-angularly spaced about the central axis and preferably the arms are identical because those characteristics permit the radial forces between the spring and the reciprocating body to have a vector sum that is a resultant of essentially zero radial force when the arms are radially centered. If the arms are off center, they apply a centering force returning an axially reciprocating body to the axis. Because of the centering forces and the absence of side forces when centered, the body is maintained on the axis but can reciprocate along the axis in resonance. It is not necessary that the arms extend along a mathematically correct spiral. In fact some flexure bearing springs have linear arms with angularly intersecting segments. The arms of flexure bearing springs extend along a path progressing radially inwardly toward the central axis and angularly about the central axis so that the arms are longer than if they were radial and therefore each arm applies a smaller radial side force than would radial arms and can translate further along the axis during reciprocation. The particular contour of the arms as they progress from the peripheral frame inwardly toward the axis is principally a matter for the judgment of the designer and often requires trial and error experimentation.
Because a flexure bearing spring functions as a spring and maintains a body attached to it on a central axis, it is often used to support and resonate the displacer piston of a free piston Stirling machine. The displacer is attached to the spring by a connecting rod and reciprocates along its central axis within the displacer cylinder at the operating frequency of the machine. The spring is also attached to the casing of the Stirling machine so that it forms a spring connected between the displacer and the casing. In order to increase the spring constant (stiffen the spring), multiple identical flexure bearing springs are stacked together and all are connected to the same connecting rod and to the casing.
For many free piston Stirling machines the power piston of the Stirling machine can be resonated using only the Stirling Cycle pressure swing so no separate spring is used to resonate the power piston. However, for some free piston Stirling machines it would be desirable to add some mechanical spring to the piston, either to statically axially center the piston, and/or to make the engine run faster at a given pressure. But in practice this is seldom done because the addition of a second spring for the power piston adds mechanical complexity, additional parts and weight and significantly increases the length to the machine. The reason is that both the displacer and the power piston reciprocate out of phase and along the same axis. Therefore care must be taken to avoid interference, such as collision, between the two springs. Consequently, with two pistons reciprocating along the same axis and each piston linked to a different flexure bearing spring, the two springs must be axially displaced apart to avoid interference between the springs. Typically the added power piston spring is axially displaced by approximately the piston amplitude away from the displacer spring to avoid interference during operation. As a result, the length of the Stirling machine must be increased by the length of the spacing between the springs. That means that the size of the outer casing must be increased as well as the length of the power piston connecting rod. This increased size and resulting increase in weight make the Stirling machine larger and heavier thereby reducing its power to weight and power to volume ratios.
It is therefore an object and feature of the invention to provide a flexure bearing spring that is capable of springing to a casing two different coaxially reciprocating bodies, such as the power piston and displacer of a free piston Stirling machine, without requiring the lengthening of the machine beyond the length required if only one body were sprung to the casing.
Another object and feature of the invention is to provide a flexure bearing spring that is capable of springing two different coaxially reciprocating bodies, such as the power piston and displacer of a free piston Stirling machine, to each other and also springing one of them to the casing.